The present invention relates to a plugged honeycomb structure and a manufacturing method therefor. More specifically, the present invention relates to a plugged honeycomb structure used as a filter for trapping or purifying particulate matter contained in exhaust gas discharged from internal combustion engines such as a diesel engine or from various combustion apparatuses and to a manufacturing method therefor.
Exhaust gas discharged from internal combustion engines such as a diesel engine or from various combustion apparatuses (hereinbelow referred to as “internal combustion engines or the like”) contains a large amount of particulate matter (hereinbelow arbitrarily referred to as “particulate matter”, “particulates” or “PM”) mainly containing soot (graphite). Since air pollution is caused when the particulate matter is released as it stands into the air, it is general that a filter for trapping particulate matter (e.g., diesel particulate filter: hereinbelow sometimes referred to as a “DPF”) is mounted in the exhaust gas flow passage from an internal combustion engine or the like.
An example of the filter used for such a purpose is a plugged honeycomb structure (hereinbelow sometimes referred to as a “honeycomb filter”) having a substrate of a honeycomb structure having a plurality of cells functioning as exhaust gas flow passages and separated and formed by porous ceramic partition walls having a large number of pores and alternately plugged with plugging portions in one side opening end portions and the other side opening end portions of the cells (see, e.g., JP-A-2006-255539, JP-A-2005-248726, and JP-A-2008-188511).
For example, the plugged honeycomb structure (honeycomb filter 110) shown in FIG. 11 is provided with a honeycomb structure 100 (honeycomb substrate) where a plurality of cells 101 are separated and formed by the porous ceramic partition walls 105 having a large number of pores, and the one side opening end portions X and the other side end portions Y of the cells 101 are alternately plugged with plugging portions 107. Such a honeycomb filter 110 is constituted in such a manner that, when gas G1 flows in from the cells 101a (hereinbelow sometimes referred to as “inflow cells”) open in the one side opening end portions X, the gas passes through the partition walls 105 from the surfaces of the partition walls 105 separating and forming the cells 101a, and the gas G2 is discharged from the cells 101b open in the other side end portions Y. That is, the gas G1 flowing into the cells 101a flows out to the cells 101b via the pores formed in the partition walls 105 and discharged from the other side end portion Y of the honeycomb filter 110. In addition, as described above, when the exhaust gas (gas G1) passes through the partition walls, particulate matter in the exhaust gas is trapped by the partition walls, and the gas is purified.
However, since the DPF has such a structure as described above, if the trapping of PM is started in a clean state, PM deposits in the pores inside the partition walls (deep bed filtration) to sometimes increase pressure drop rapidly. Such rapid increase in pressure drop could be a factor of deterioration in engine performance. In order to solve this problem and improve PM trapping efficiency, there is disclosed a DPF where a trapping layers are formed on the inflow side surfaces of the partition walls to inhibit PM from entering the inside of the partition walls (see, e.g., JP-Y-2607898).
In addition, as a method for manufacturing such a honeycomb filter, for example, there is proposed a method where microparticles of a ceramic material to constitute the aforementioned filter layer is sent into one side face of a ceramic porous support by means of a gas current to allow the microparticles to adhere to the one side face of the porous support, and moisture is imparted to the adhering microparticles to allow the microparticles to be absorbed on the one side face of the porous substrate (see, e.g., JP-A-10-249124).
In addition, there has conventionally arisen a problem of gradually clogging the opening portions of the inflow cells by particulate matter in exhaust gas which deposits on the inflow side end face of plugging portions and is enlarged by aggregation of the particles to develop up to the opening portions of the inflow cells. Therefore, there has been proposed also, for example, a plugged honeycomb structure constituted in such a manner that the plugging members have protruding portions protruding outside from the opening end faces of the cells (see, e.g., JP-A-2004-344722 and JP-A-2003-176709).
However, in a plugged honeycomb structure as shown in JP-Y-2607898, exhaust gas blows against the plugging portion end face to cause disorder of the gas flow in the vicinity of the inflow side opening portions. This inhibits soot from entering the inside of the cells and causes deposition in the inflow side opening portions. This reduces the inlet opening diameter to cause a problem of deterioration in pressure drop properties.
In addition, in a plugged honeycomb structure as shown in JP-A-2004-344722 and JP-A-2003-176709, the inflow side end face of the plugging portions are intentionally protruded upon forming plugging portions, or protruding portions are disposed separately on the inflow side end faces of the plugging portions. However, there are problems that the process for manufacturing the protruding portions are very complex, that the shape of the protruding portions obtained is not uniform because the plugging material used is in a slurried form, and that sufficient effect cannot be obtained even when the aforementioned protruding portions are provided because it is very difficult to form the shape as intended since the protruding portions are very minute.
In addition, since the protruding portions formed later of the plugging portions are connected only to the end faces of the plugging portions, the connection strength of the protruding portions disposed on the end faces of the plugging portions is very weak, and there sometimes arises a problem of detachment of the protruding portion from the end face of the plugging portion or breakage of the protruding portion due to wind pressure of exhaust gas, vibrations transmitted to the honeycomb structure, or thermal shock.